Conventionally, in copying machines and the like that use an electrophotographic method, an electrostatic latent image is formed on a photosensitive drum, followed by converting the resulting latent image into a toner image, transferring the toner image to a transfer material such as paper, and fixing the toner on the material such as paper by applying heat, pressure, or the like to obtain a copy or record image.
In such copying machines and the like, a toner image formed by development on the photosensitive drum is transferred onto transfer material and then the transfer material carrying the toner image is passed between a fixing roller and a pressure roller pressed against each other while rotating so that the toner image is fusion-bonded to the transfer material by the effect of the heat and pressure of the fixing roller.
The above fixing roller is equipped with a fixing belt and this fixing belt can suffer the adhesion of residual toner left unfixed on the transfer material or powder of the paper used as transfer material, possibly causing failure in the fixing of a new toner image. Therefore, it is necessary to perform continuous cleaning to remove the toner and paper powder adhered on the fixing belt. If not equipped with a fixing belt, the fixing roller has to be cleaned continuously to remove the adhered toner and paper powder as described above.
A method of removing toner or the like such as described above is to supply a toner cleaning sheet such as nonwoven fabric wound into a roll and allow the sheet and the fixing belt to pass together between a toner cleaning heating roller and a pressure roller to heat them under pressure so that the toner and paper powder adhered on the fixing belt are transferred to the sheet, thereby removing the toner and the like from the fixing belt.
Useful materials for the toner cleaning sheet include woven fabrics, knitted fabrics, and nonwoven fabrics formed of polyester fiber, nylon fiber, cellulose fiber, polyethylene fiber, polypropylene fiber, rayon fiber, vinylon fiber, pulp fiber or the like (see Japanese Unexamined Patent Publication (Kokai) No. HEI 10-116011).
In this case, the toner cleaning heating roller is usually operated at a temperature about 180° C. to 200° C. and, accordingly, the toner cleaning sheet has to be resistant to a temperature at around 180° C. to 200° C. Moreover, when a copying machine is activated from the standby state to the functional state for printing, the temperature of the toner cleaning heating roller may exceed (i.e., overshoot) the specified temperature momentarily and reach about 230° C. as it has to be increased very rapidly. Therefore, the toner cleaning sheet is required to retain strength to resist instantaneous heating up to a temperature about 230° C. Furthermore, the copying machine should be high in cleaning performance because if cleaning performance is high for this operation, it serves to shorten the length of the toner cleaning sheet to be installed in the copying machine, thus enabling space saving (i.e., reduction in copying machine size).
On the other hand, a paper product that contains polyphenylene sulfide fiber has been proposed as a material for sheets with increased heat resistance (see Japanese Unexamined Patent Publication (Kokai) No. 2012-127018).
In addition to the above sheet, another toner cleaning sheet that contains aramid fiber has also been proposed (see Japanese Unexamined Patent Publication (Kokai) HEI 7-287496). Aramid fiber is a synthetic fiber composed of amide bonds (—NHOC—) that connect aromatic rings such as benzene rings to form a macromolecular polyamide, which is also called aromatic polyamide. Among others, meta-aramid fiber has high heat resistance.
However, conventional toner cleaning sheets such as the one described in JP '011 are not sufficiently high in heat resistance and cannot be said to show satisfactory performance.
Furthermore, if a PPS fiber based sheet as described in JP '018 is used for toner cleaning, the sheet may be softened as the heating roller for toner cleaning reaches a temperature of about 230° C. as a result of overshooting, causing elongation of the sheet and leading to poor practicality.
In addition, toner cleaning sheets formed of meta-aramid fiber as described in JP '496 are high in cost and have problems related to material availability.
Thus, it could be helpful to provide a toner cleaning sheet high in heat resistance, high in durability with little decrease in tensile break strength under heat even in temperature overshoot in heating rollers for toner cleaning, high in cleaning performance, and low in price.